A conventional display device of the type for displaying alpha numeric characters is composed of as shown in FIG. 1, a light reflecting block 1 having a plurality of light paths 2 for making up respective segments of the alpha numeric character with their uppermost openings 3, light emitting diodes 4 each of which is disposed on a substrate 5 within each of the light paths 2 for illuminating corresponding uppermost openings 3 upon activation of the light emitting diode 4 and a light diffusion sheet 6 laid on the light reflecting block 1. However, the light absorvability of the light diffusion sheet 6 laid on the light reflecting block 1 is so high that, when a light emitting diode is electrically activated, the corresponding segment is illuminated dark as compared to the intensity of light emitted by the light emitting diode. In order to increase the intensity of the light illuminated by the segment, a lens sheet 7 is employed in place of the light diffusion sheet 6. Said lens 7 sheet is formed with lenticular lens parts in respective positions opposed to the uppermost openings 3 for making the display of the segments uniform.
A display device of the type is disclosed in U.S. Pat. No. 3,786,499.
In the conventional display device as mentioned above, there is a disadvantage that both end portions of the lengthwise direction of a segment tend to become dark relative to the intermediate part of the segment when the segment is illuminated upon activation of the corresponding light emitting diode.
According to the conventional display device of the type mentioned above, the image of the light made up on one segment tends to include a series of lighting spots lined up in the lengthwise direction thereof due to the lens effect of every lens columns of the lenticular lens. Under such condition, since the light emitting area of the light emitting diode is so tiny that respective sizes of the light emitting spots 8 appearing on both end portions of one segment tend to become smaller as shown in FIG. 2 compared to those of the lighting spots displayed on the intermediate portion of the segment, thereby resulting in unsightly display of a character.
On the other hand, a Fresnel lens is used in place of the lenticular lens, the shape of image of the segment displayed varies corresponding to the view angle of a viewer relative to the segment. In addition, to make up a desired image of the illuminated segment, it is necessary to adjust the focus of the Fresnel lens relative to the light emitting diode. But the work of adjusting the focus is difficult and time consuming.